1. Field of the Invention
The invention relates to a method and to a device for removing the load from a seed crystal during pulling according to the Czochralski method, more particularly to a method and device for support a crystal ingot while pulling a single crystal according to the Czochralski method.
2. Background Art
Typically, in the actual pulling process utilizing the Czochralski method, a neck with a sharply tapered cross section is first grown in order to minimize to the utmost extent structural defects such as subgrain boundaries or dislocations arising from melting or imparted by the seed crystal, by a type of xe2x80x9coutgrowthxe2x80x9d from the neck of the growing crystal.
The growth is then regulated so that a conical section is followed by a cylindrical section with a much larger, essentially uniform diameter, followed at the end by another conical section. Only the cylindrical section is typically of interest for use in the electronics industry, for example. There is a user demand for ever-larger diameters and, thus, for substances from these cylindrical sections (with diameters of 300-400 mm and greater and weights of 250-400 kg and greater).
The technical and technological problem lies in the fact that the neck, which is kept very thin by growing techniques, cannot bear these tensile loads. A further problem lies in the fact that in certain cases at least partial remelting of the newly grown crystal must be performed, thus creating the need for a support solution by internally releasing the support on the crystal neck at any time. The process for supporting the crystal neck must proceed very slowly and controllably during transmission of the load from the neck to the support device. This is because no particles of any type, such as from abrasion, can be allowed in the melt since such contaminants would disrupt crystal formation as well as the solidification process at the melt front. In addition, transmission of the load must have absolutely no influence on the actual pulling speed, or else uncontrolled crystal growth would ensue.
In U.S. Pat. No. 4,973,518 it has been noted that during pulling of a single crystal, the thin neck can be subjected to an axial load, but that any transverse forces must be avoided since the slightest lateral stress can break off the neck. In order to avoid this hazard, it was proposed to expand the thin neck in the axial direction at a specified position so as to form a bead to bear the load, with a bearing device resting on said bead. A similar design is proposed in U.S. Pat. No. 5,126,113, but here it is not possible to disengage the bearing for the single crystal, thus leaving the problem of remelting unsolved.
Another fundamental principle of crystal support during the pulling process is proposed in German Patent Application 197 37 605 A1, whereby a permanent adhesive bond is created between the conical section of the crystal ingot at the beginning of the single crystal and a specially designed support element. Although this solves the problem of transmitting force to a specialized pulling device from the single crystal to be grown, whereby the thin neck is relieved of tensile stress during the pulling process, this bond cannot be released at will and remelting is therefore not possible.
The present invention relates to a method and to a device for supporting a crystal ingot while pulling a single crystal according to the Czochralski method. To this end, a crystal support is provided which engages with a specialized bead, which is formed on the neck of the single crystal ingot and which preferably has the shape of a bicone, by means of bearings mounted in a housing. The housing is connected to a second pulling element. The bearings within the housing are moved into a support position on the specialized bead by a central pulling element which can be independently displaced relative to the second pulling element. Support of the crystal ingot is thus achieved which may be disengaged at any time, and which has no disruptive effects on crystal growth and which acts independently of the length of the grown crystal.